In-Cylinder Control of Smoke and NOx by High Turbulent Two-Stage Combustion in Diesel Engines 962113
The authors have previously reported significant reductions in particulate emissions by generating strong turbulence during the combustion process. Extending this, it was attempted to reduce NOx, particulate, and fuel consumption simultaneously by two-stage combustion: forming a fuel rich mixture at the initial combustion stage to prevent NOx formation, and inducing strong turbulence in the combustion chamber at the later stage of combustion to oxidize the particulate. The purpose of this study is to examine the effect of two-stage combustion in emission control. The paper gives an evaluation of the NO reaction-kinetics of the system and experimental results for a combustion chamber specially made for the two-stage combustion. With this combustion system, it was possible to reduce NOx levels to 1/3 of the base engine. Combination of EGR and the two-stage combustion was also examined.
REDUCTIONS in smoke and NOx emitted from diesel engines is a major concern in engine research, and there are many studies on the control of these emissions. Other than EGR and water injection, the factors which can be controlled and are important in reducing emissions are the mixture concentration and mixing intensity.
The authors previously reported significant reductions of particulate emissions by generating strong turbulence during the combustion process (1, 2). The strong turbulence was generated by injecting a small amount of fuel into an auxiliary chamber installed in the cylinder head during the main combustion. The system has good combustion control characteristics because the intensity and timing of the turbulence can be controlled separately. The authors extended this system to a two-stage combustion concept, where the primary combustion is made fuel-rich to reduce NOx, and the increased particulate is oxidized by strong turbulence generated during secondary combustion (3, 4). The present research was conducted to examine the possibility of further reductions of emissions with a combustion chamber specially made to take advantage of this system. Additionally, a combination of this system and EGR was investigated.
Kamimoto et al. attempted to adapt an air cell to a direct injection diesel engine in 1984 to activate the oxidation of soot (5). In 1988, the authors here first presented the high turbulence system at the Japan IC engine symposium, Tokyo (6). That report was presented in English at the CIMAC congress in 1989 (1). Following this, Yamaura et al. conducted basic research to determine the effects of turbulence on smoke reduction, by injecting various gases into the combustion chamber (7). They showed that the kinetic energy of the gas jet and the kind of gas are important factors in the smoke reduction.
In 1990 Kawazoe et al. reported a plunger type air-jet generator (8), where the plunger is in an air cell compressing air to generate a turbulent jet. Nagano et al., from the same research group at Toyota Central Research, reported a spring-accumulated air-jet generator (SAAG) to counter the power loss with the plunger type air-jet generator in 1991 (9). Here a plunger with a spring is actuated by high pressure gas from the main chamber during the compression stroke, and as the piston goes down, the plunger pushes air into the main chamber, generating turbulence.
Lu et al. have also developed a unique turbulence generating chamber with micro-chambers in the piston cavity, where some of the fuel jet is injected into the chamber (10). This work was done mainly to achieve improvements in the autoignition of low cetane-number fuels, rather than to achieve smoke reduction.
Pierpont et al. developed split injection system, where fuel spray was injected in multiple steps from a nozzle (11). The experimental result showed apparent smoke reduction due to increased mixing and air entrainment with the split injection.
The authors here continued research on the high turbulence system, and presented additional results with details of the smoke reduction mechanism in 1992 (2). Then, using
the system with a high degree of freedom in the control of turbulence, two-stage combustion was applied to DI diesel engines for further simultaneous reductions in NOx and particulate emissions (3, 4). In addition to the experimental research, the authors investigated the effects of two-stage combustion and its major parameters on NOx reductions with chemical reaction kinetics (12).
In this research a modified OSKA type combustion chamber (13) was applied to the two-stage combustion system. The piston has a smaller cavity volume than the standard piston and has an air cavity which reserves air necessary for the secondary combustion. The results of the experiment with the piston showed the NOx level to be 1/3 of the base engine.